China is demanding the release of telecoms giant Huawei's chief financial officer, who has been detained in Canada.
Meng Wanzhou, the daughter of the company's founder, could face extradition to the US.
She was arrested in Vancouver on 1 December, but the news was not made public at her request.
The charges remain unknown but the US has been probing Huawei over possible violation of sanctions against Iran.
China says her detention is possibly a rights abuse.
Ms Meng has sought a publication ban on the details of the arrest, which has been granted by the courts.
Huawei said it was "not aware of any wrongdoing by Ms Meng".
Huawei is one of the largest telecommunications equipment and services providers in the world, recently passing Apple to become the second-biggest smartphone maker after Samsung.
European shares hit a two-year low and indexes across Asia dropped sharply following the arrest, which analysts said revived worries over the US-China trade war.
How has China responded?
A Chinese foreign ministry spokesperson told reporters: "The detention without giving any reason violates a person's human rights."
"We have made solemn representations to Canada and the US, demanding that both parties immediately clarify the reasons for the detention, and immediately release the detainee to protect the person's legal rights."
Beijing has itself frequently been accused by rights groups of rights abuses including unexplained detentions.
However, concerns the arrest would affect the 90-day tariff truce negotiated between the two nations at the G20 have not yet materialised. China announced in a regular press briefing on Thursday that it would "immediately" implement the measures agreed.
It also coincides with moves to restrict the use of Huawei technology in Western countries. The US, Australia and New Zealand have blocked the use of the Chinese firm's equipment in infrastructure for new faster 5G mobile networks.
Canada's ministry of justice said a bail hearing for Ms Meng had been set for Friday.
The gloves are off
By Karishma Vaswani, BBC Asia Business Correspondent
It is hard to overstate the symbolism and significance of this event. Huawei is the crown jewel of Chinese tech and Ms Meng is effectively its princess.
Even though it's still not clear what the charges against her are, this is not simply a case about the arrest of one woman, or just one company.
This arrest could materially damage the relationship between the US and China at possibly one of the most sensitive times between the two countries in their long and torrid history.
The gloves are off. Things have taken a dramatic turn for the worse.
What could be behind it?
US media reported that Huawei is under investigation for potential violations of US sanctions against Iran.
One report in the New York Times said the US commerce and treasury departments had subpoenaed the firm over suspected violation of sanctions against both Iran and North Korea.
US lawmakers have repeatedly accused the company of being a threat to US national security, arguing that its technology could be used for spying by the Chinese government.
Meanwhile, US National Security Adviser John Bolton said on Thursday that he had known in advance about Ms Meng's arrest.
He told National Public Radio that he didn't know if President Donald Trump had also been aware.
In a statement, Huawei said it had complied with "all applicable laws and regulations where it operates, including applicable export control and sanction laws and regulations of the UN, US and EU."
The arrest is a top trending topic on Chinese social media, BBC Monitoring says, with many users criticising the US and Canada for what they call "low" and "bullying" tactics.
Why is Huawei a concern to the West?
Some Western governments fear Beijing will gain access to fifth-generation (5G) mobile and other communications networks through Huawei and expand its spying ability, although the firm insists there is no government control.
The US has brought a number of legal cases against Chinese technology firms, with accusations such as cyber-security theft and violations of Iran sanctions.
Donald Trump last month reinstated all the US sanctions on Iran that had been removed under a 2015 nuclear deal.
Mr Trump had been fiercely opposed to the deal, which saw Iran limit its controversial nuclear activities in exchange for sanctions relief.